Current ink jet printing techniques use printheads capable of ejecting drops of ink or other fluids onto a media to generate a printed image. Common printhead technology includes thermal inkjet (TIJ) printheads and piezo-electric printheads.
Many ink jet printing systems use four primary colour inks—cyan (C), magenta (M), yellow (Y), and black (K)—in a so-called CMYK colour model.
Digital images, for example as produced by a digital camera or as represented on a visual display unit of a computing device, however, typically use different colour models, such as the red, green, and blue (RGB) colour model. In an RGB image each pixel of the image is represented by a 3-tuple (or triplet) of multi-bit values, one for each of red, green, and blue colours or channels. Many colour image formats represent each image pixel using 8 bits per colour channel. This enables 256 shades (or grey levels) to be represented per colour channel per pixel, allowing each image pixel to represent one of 2563 (over 16 million) possible colours.
Ink jet printheads, however, are typically only able to represent 2 grey levels per colour on a media. That is, an ink jet printhead may either eject a drop of ink onto the media, or may not eject a drop of ink onto the media. Two grey levels may be represented by just a single bit.
In order to authentically reproduce multi-bit or continuous tone images with a printer having only 4 primary ink colours, images to be printed have to be in the CMYK colour space. Images not in the CMYK colour thus have to be converted using known conversion techniques.
A halftoning process is then performed on the CMYK image data to produce, for each of the 4 primary colours, a 1-bit image known as a halftone screen. A halftone screen defines, for a particular colour ink, a two-dimensional array of spatial locations at which ink jet drops are to be placed (and hence ink marks made) on a media. The collective result of each halftone screen being printed by its associated colour printhead creates a visual illusion, when viewed at a suitable viewing distance, of continuous tone colour images.
If these ink drops can be printed at high resolution, using very small drops, such halftoning techniques can produce very high quality prints (for example, even so-called ‘photo quality’ prints) even when viewed from close-up. This is typically the case with small format consumer printers.
However, in large format or industrial inkjet printers the size of ink drops ejectable from a printhead are somewhat larger than those ejectable from a printhead of a typical consumer printer. For example, an industrial inkjet printer may eject drops having a volume in the range of about 35 to 160 pl, whereas office or consumer inkjet printers may eject drops having a volume in the range of about 6 to 15 pl. This is due partly because of the large numbers of print nozzles used in large format or industrial printers for printing on large widths of media. Accordingly, large format or industrial printers using of a four colour printing process may produce images that exhibit grain when viewed close-up, especially in photographic or other images having relatively light tones. Grain in light tones is due to the halftoning process having to increase the spacing between ink drops such that the combination of the media background colour (generally chosen to be white) and the printed drops creates the visual effect of a light colour tone.
In order to improve the quality of such images, to reduce grain, and provide smooth colour transitions, some large format or industrial printing systems use additional lighter colour inks in addition to the four primary CMYK colour inks. For example, some printing systems may use an additional light cyan (c) ink and a light magenta (m) ink, in a so-called CcMmYK or 6-colour model. Light yellow is typically not used as the human eye is less sensitive to light yellow. In some examples a light black (k) ink may also be used. Typically, a light colour ink will have a lower density or concentration of colorant compared to its equivalent full colour ink. In such systems, the halftoning process is extended to generate halftone screens for each of the ink colours (both full and light colour inks) available to the printing system. Thus, in a six colour printing system, six separate halftone screens would be produced.
Halftoning is well understood for use with printing systems such as those described above, and a great deal of research has been invested in deriving efficient halftoning techniques that provide high quality printed output that exhibit low noise, interference, and the like in printed images. Accordingly, halftoning technology, commonly implemented in raster image processors (RIP), is generally available at low cost and reliable.
Some modern printheads are also capable of producing ink drops of varying sizes. Combined with 4 or 6 (or greater) colour inks, such printheads may enable a significant increase in the number of grey levels producible by a printer. However, to maximize the potential of such printheads, new halftoning processes have to be designed and implemented, which is a non-trivial task.